This invention relates to a method of fabrication for multi-step microlithographic structures. More particularly, the invention is directed to a method of fabrication for multi-step microlithographic structures (e.g. Fresnel lenses used in acoustic ink printing applications) whereby the process includes the formation of intermediate etch stop layers within the block from which the structure is formed. In one embodiment, the method includes forming such layers by depositing etch material in-situ with Fresnel lens material. This is accomplished in the invention by depositing the lens material using known techniques and then selectively altering the chemistry of the material being deposited to form the intermediate etch stop layers at suitable positions without interrupting the deposition process. In another embodiment, the etch stop layers are patterned between layers of lens material. In either case, the lens is then formed using masking, patterning and etching techniques.
While the invention is particularly directed to the art of fabrication of multi-step microlithographic structures including, as a specific example, Fresnel lens fabrication in the acoustic ink printing environment, and will thus be described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. In this regard, the fabrication method also applies to Fresnel lenses or other multi-step microlithographic structures for use in, for example, optical, mechanical and electromechanical devices and is not limited to the acoustic ink printing environment.
Nonetheless, multi-discrete-phase Fresnel elements have been proposed for optical applications. See Swanson et al., "Infrared Applications of Diffractive Optical Elements," Holographic Optics: Design and Application, SPIE Vol. 883, 1988, pp. 155-162. Indeed, Fresnel lens elements have been proposed for use specifically in connection with acoustic ink printing applications. In this regard, commonly assigned U.S. Pat. No. 5,041,849 to Quate et al., which is incorporated herein by this reference, discloses the use of Fresnel lenses in acoustic ink printing systems. In addition, commonly assigned U.S. Pat. No. 5,278,028 to Hadimioglu et al. and commonly assigned U.S. Pat. No. 5,779,751 to Wong, which are also incorporated herein by reference, disclose processes for fabricating Fresnel lenses.
To specifically demonstrate an example of an environment of use for a Fresnel lens, FIG. 1 shows a portion of an exemplary acoustic ink printhead 11 comprising an array of substantially identical multi-discrete-phase binary Fresnel acoustic focussing lenses 12a-d. This particular printhead configuration is well suited for certain types of printing, such as line printing, but it should be appreciated that such printheads (or substantially similar printheads) are also applicable to other pertinent configurations for implementing a variety of different print modes.
As shown in FIG. 2, the printhead 11 is embodied in an acoustic ink printer 13 for ejecting individual droplets of ink 14 from a free surface 15 of a pool of liquid ink 16 on demand at a sufficient ejection velocity to cause the droplets 14 to deposit in an image configuration on a nearby recording medium 17. To that end, the printhead 11 comprises a planar piezoelectric transducer 21 which is disposed on the rear face of an acoustically conductive substrate 22, such as a acoustically flat quartz, glass or silicon substrate. The opposite face of the substrate 22 has disposed thereon the concentric phase profiles of the Fresnel lens 12a which is generally representative of all lenses used. Sound waves are generated by the transducer 21 and focussed by the Fresnel lens toward the surface 15 of ink pool 16 to emit the droplets 14 therefrom.
As to the Fresnel lens, it should be appreciated that the representative lens 12a is a quantized approximation of the continuous phase profile of a theoretically ideal, 100% efficient, Fresnel phase plate. Accordingly, it will be evident that the acoustic focussing efficiency of the lens 12a and the width of its narrowest feature are dependent upon the number, n, of discrete phase levels to which its phase profiles is quantized. More specifically, as described in the above identified Swanson et al. article, two phase, four phase, eight phase and sixteen phase embodiments are approximately 41%, 81%, 95% and 99% efficient, respectively, for diffracting axial incident radiation into a focussed +1 diffraction order. A four phase Fresnel lens that is approximately 81% efficient has been determined to be acceptable for most acoustic ink printing applications.
There is a need for an economical and reliable process for manufacturing Fresnel lenses of the type noted herein that are in compliance with the design specifications of acoustic ink printing, for example. In this regard, typically, a four-phase Fresnel lens structure consists of three discrete steps. Depending on the process used, the thickness of each step may be determined by a time etch. In this case, uniformity may be compromised due to nonuniform etching characteristics resulting in over-etching and under-etching. Therefore, uniform etching is desired because overall ejector uniformity and Fresnel lens uniformity is critical to good print quality. Likewise, it is to be appreciated that uniformity and an economic and reliable process are also desired in applications other than Fresnel lens fabrication for acoustic ink printing.
The present invention contemplates a new and improved fabrication method for multi-step microlithographic structures including Fresnel lenses used in, for example, acoustic ink printing applications and other such structures used in other applications which resolves the above referenced difficulties and others.